1. Field of the Invention
This invention relates to a multi-beam generator which splits light, for example laser light, into a plurality of beams which are used independently and simultaneously to realize high-speed operation. In particular, the invention is a multi-beam generator that can effectively be used with an optical apparatus that splits laser light into a plurality of beams, thereby converting the profile of illumination with the laser light to a desired pattern.
2. Description of the Related Art
When laser light falls incident on a grating device, a number of beams of diffracted light will be produced. It is known that this phenomenon can be utilized to apply a grating device as a multi-beam generator. The term "multi-beam" or sometimes "multiple beams" as used herein shall apply to the generation of three or more light beams.
FIG. 2 shows an example of optics that utilize a grating. The grating 7 is formed on a substrate 2. When the grating 7 is illuminated with laser light 1, a number of beams of diffracted light 5 are produced. If the diffracted light 5 is observed with the grating 7 being positioned at the front focal point of a lens 3 while a screen or the like is positioned at the rear focal point 4 of the same lens, a number of spots appear at equal spacings on the screen. In this way, a single light beam can theoretically be converted to multiple beams but, in practice, it has been difficult to efficiently generate multiple beams of uniform intensity.
The grating device for converting a single light beam to a plurality of beamlets, described by G. Hatakoshi and M. Nakamura on pages 84-87 of the Technical Digest for the Third Microoptics Conference held in 1991 at Yokohama, Japan, has been one of the approaches so far taken to solve the aforementioned problem. As shown in FIG. 16, a substrate 35 has a grating 36 composed of a reference phase pattern formed thereon. The grating 36 is further segmented at given widths that are furnished with another phase pattern that consists of three or more levels. This approach has proved successful to some extent in achieving uniformity in the intensity of multiple beams.
However, this approach requires the additional phase pattern to be formed at multiple levels, so it is necessary to perform a difficult process in which a material for electron beam (EB) exposure is illuminated with EBs as their intensity is controlled precisely at multiple levels. As a further problem, the fabrication process limits the number of multiple levels that can be imparted to the additional phase pattern and, hence, there are also limits on the number of multiple beams generated, the efficiency of light utilization and the uniformity in the intensity of the multiple beams.